An optical fiber cable used for communications includes an optical fiber. The optical fiber has a core that communicates light and cladding that surrounds and protects the core. The cladding is in turn covered with one or more coating, insulating, shielding, and/or support layers to complete the cable. Considering that a typical optical fiber core may measure only about 8 to 50 microns in diameter, the connection of two optical fiber cables so that their cores are precisely aligned is a formidable task.
In order to establish such a precise connection between optical fibers to be coupled, several different connection configurations have been developed in the art. One known configuration for establishing a connection between optical fibers is referred to as a ferrule connection. An example of a ferrule connection is shown and described in both U.S. Pat. Nos. 4,738,508 and 4,738,507 to Palmquist. Another known configuration is referred to as a biconic connection. An example of a biconic connection is shown and described in U.S. Pat. No. 4,787,698 to Lyons et al.
In both of the aforementioned connection configurations, the ends of the optical fiber cables to be joined are terminated, and the terminations are provided with a surrounding support material, or plug. To enable optimal performance and minimize light energy losses and reflections, the termination endfaces of the separate optical fiber terminations should be coupled so that the cores of the respective endfaces are precisely aligned. To achieve this end, the termination endfaces are joined by a coupling structure, which engages and aligns respective alignment surfaces on the corresponding termination plugs. In the case of a ferrule termination, the alignment surface is a generally cylindrical outermost boundary surface of the plug, which measures about 2500 microns in diameter. Moreover, in the case of a biconic termination, the alignment surface is a bevelled chamfer with an endface measuring about 4400 microns.
Successful assembly of a fiber termination for one of the aforementioned connections requires that the endface of the core be disposed very close to the center of the termination endface. The core endface (about 8 to 50 microns in diameter) is much smaller in diameter than the termination alignment surface (about 2500 microns in the case of a ferrule termination and about 4400 microns in the case of a biconic termination). Moreover, the offset, or eccentricity, of the core relative to the alignment surface should not exceed a micron on each of the two mating terminations. To achieve the foregoing precision, it is desirable to measure the eccentricity within a precision of about 0.1 micron. Several known prior art methods for measuring the eccentricity are described hereafter.
A first method involves digitizing points on the boundary of the core endface and of the termination endface by moving the termination endface relative to a toolmaker's microscope with the termination axis disposed parallel to the viewing axis of the toolmaker's microscope. Mathematical equations are then fitted to the digitized points in order to determine the centers of the core and termination. The distance between these two center points is defined as the eccentricity.
A second method of measuring the eccentricity involves viewing the core under a high power microscope, while the termination is rotated in a fixture, such as a V-shaped support block. The movement of the core is measured as the termination is rotated about its longitudinal axis. The locus of points defining the center of the core is, in general, circular as the termination is rotated, and the radius of the circle is equal to the eccentricity. The aforementioned technique is described in more detail in U.S. Pat. No. 4,787,698 to Lyons et al., relative to a biconic connection, and in U.S. Pat. No. 4,738,508 to Palmquist, relative to a ferrule connection.
A third method of measuring the eccentricity focuses upon measuring the effect of eccentricity and involves interconnecting the termination under test to a reference connector, sometimes referred to as a "golden connector," that is known to have negligible eccentricity. After establishing the connection with a coupling structure, the light transmission therethrough is measured. The eccentricity is determined based upon the loss of light and one or more mathematical equations that define the light loss as a function of the eccentricity.
A fourth method, which is basically obvious but has not been demonstrated successfully to date for reasons set forth hereafter, involves obtaining an image of the entire termination endface and fitting points to the boundaries of the core and termination. After generating the foregoing image, the offset can then be directly computed using known mathematical techniques. For instance, the offset can be determined by first fitting circles to the boundary pixels and core pixels, respectively, then determining the circle centers, and finally, the offset can be calculated as the displacement between the circle centers.
However, at present, the foregoing method cannot be practically implemented because of the extreme disparity in size between the core endface (about 8 microns in diameter) and the termination endface (2500 microns in diameter). Unfortunately, conventional machine vision systems have a standard resolution of typically 512.times.512 picture elements (pixels) and thus would have inadequate resolution to precisely locate the core with the desired resolution. More specifically, with the termination endface measuring 2500 microns in diameter, each pixel would represent about 5 microns. The core, with a diameter of for instance about 8 microns, would span only 1 to 2 pixels, and consequently, the process of locating the core to the requisite precision of 0.1 micron would be impossible. Locating the termination to a precision of 0.1 micron would require a subpixel resolution of about 1/50th of a pixel, which is generally not considered to be a routinely achievable practice in the machine vision art. Moreover, if an image of the termination endface is magnified to the extent that the core represents an adequate number of pixels, the termination boundary would no longer be present in the image and its position cannot be ascertained with accuracy. Furthermore, to further complicate imaging, the termination boundary and core boundary are not generally coplanar.
Hence, a heretofore unaddressed need exists in the industry for a system and method of contactlessly measuring the eccentricity of an optical fiber termination that is accurate within at least 0.1 micron, is less labor intensive, less material intensive, and less expensive than presently known methods. More particularly, a system and method are needed for contactlessly measuring the eccentricity of an optical fiber termination to a precision of at least 0.1 micron without the requirement of moving the termination on a coordinate measuring system such as a toolmaker's microscope, without the requirement of rotating the termination about its longitudinal axis to observe relative core movement, and without the requirement of connecting the termination to a transmission measurement test set in order to measure the light loss caused by misalignment.